The present invention relates to a system for a four-wheel drive vehicle in which the power transmission is automatically changed from two-wheel drive to four-wheel drive or vice versa in accordance with driving conditions.
Japanese patent laid open 55-4292 discloses a four-wheel drive vehicle. The four-wheel drive vehicle is provided with a transfer clutch for transmitting the power of an engine to two auxiliary driving wheels for establishing four-wheel drive.
There has been proposed a four-wheel drive vehicle, a power transmission system of which is automatically changed from four-wheel driving to two-wheel driving in accordance with driving conditions.
Such a four-wheel drive vehicle must be driven with two-wheel driving when turning a sharp corner on paved dry surfaces. the reason is as follows. When the vehicle negotiates corners, the front wheels run through an arc of greater radius than that of the rear wheels and therefore tend to rotate faster than the rear wheels. Such a difference between the speed of the front and rear wheels causes braking of the vehicle, which is called "tight corner braking". In order to prevent such a braking phenomenon, a four-wheel drive vehicle having a transmission system which is automatically changed to two-wheel drive at a large steering angle has been proposed. However, such a system is complicated in construction.
In addition, the tight corner braking has a great effect on the vehicle when the vehicle turns a corner at low speed. Accordingly, the decision on two-wheel driving should be made not only by steering angle, but also by vehicle speed.